Follow the prompts to find Varric, he'll be more agreeable than his brother and tell you how you can join the expedition. He'll join you after you agree to his plan. Before following the map prompt to Aveline, go to the exclamation point to talk to Worthy. Doing so will unlock crafting.

Follow the minimap south to enter the Viscount's Keep and talk to Aveline. Accept the job she offers to help with the patrol. Go to the Sundermount Ambush Site and you'll be attacked almost immediately. Follow the map prompt to find all the raiders and finish them off. Beware, at the end you'll be attacked by a larger party, just remember not to let them surround you.

After the fight be sure to loot all the raider bodies before going back to the Viscount's Keep. You'll hear that the captain is not very happy and Aveline wonders why. This will lead to your next quest, which is coming to the rescue of Dominic in the Lowtown night patrol.

You'll be attacked not far into Lowtown, follow the map and fight your way through the highwaymen to find and rescue Donnic. Loot the bodies of the dead after the cutscene. Aveline will expose what the captain did no matter what you say in response.

Follow the map to return to the Viscount's Keep and find Aveline. Once you get there the captain will be in the process of being dragged away as a prisoner. A representative of the Viscount will talk to Aveline and say she'll eventually be promoted to captain of the guard.